In a heating appliance having a non-high frequency dielectric heater means, such as an electric resistance heater, and a dielectric heating means, it is very useful to rotate the antenna of the dielectric heating means. In dielectric heating, where the high frequency energy which is radiated varies from time to time, rotation of the antenna ensures a uniform distribution of the electric field. The antenna is normally in the shape of a rod and does not obstruct the heat being given off by the resistance heater means and the temperature distribution in the heating chamber is not disturbed even when the antenna is rotated between the resistance heater means and the position of the object to be heated.
However, when the resistance heater is above the object to be heated and an L-shaped antenna has the horizontal arm below the resistance heater means and rotated around the vertical arm as the rotational axis, the antenna may come in contact with the object to be heated and stop rotating and cause uneven heating, which offsets the advantage of this arrangement that it improves the high frequency electric field distribution and high frequency output level. Moreover, since the antenna may be damaged when contacted by the operator's hand or the object to be heated when the object to be heated is introduced into or removed from the heating chamber, it should be made of a sufficiently hard material that it is not damaged. Accordingly, it becomes relatively expensive.
On the contrary, when the horizontal arm of the L-shaped antenna is rotated in a plane above the resistance heater means, it is disadvantageous in that the length of the horizontal arm must be shorter and high frequency output level is reduced, especially in a small sized appliance, even though the heater means provides protection for preventing the antenna from being contacted by the object being heated.
If the horizontal arm of the antenna is flush with the resistance heater means, then the length thereof is even further limited. Where the dimension of the antenna is made such that good dielectric heating is achieved, uneven heating occurs when the resistance heater means is in operation.
It is an object of the present invention to provide an energy supplying structure which avoids the disadvantage of causing deterioration of the heating performances of both the resistance heater means and the dielectric heating means, of occupying a large space in the upper portion of the heating chamber so as to reduce the effective volume of the heating chamber of having the rotary antenna damaged by being contacted by the operator's hand or the object to be heated when the object is being introduced into and removed from the heating chamber, and of obstructing the rotation of the rotary antenna during the heating operation. This object is accomplished by rotating the antenna for radiating high frequency energy at a level above the resistance heater means which is located in an upper portion of the heating chamber.
The energy supply construction according to the present invention has an L-shaped antenna with a vertical arm having a dimension such that it operates as a so-called 1/4 wavelength dipole antenna, and with a horizontal arm disposed between a resistance heater means and the top wall of the heating chamber.